Do you eat like it’s summer all year? It could confuse your internal clock

Our blood pressure rises in the morning, our brain releases sleep hormones before bedtime, and our body temperature drops during sleep. These are all regulated by our internal clock – the human circadian rhythm – which ensures that bodily functions are timed to meet bodily needs at the right time of day.

The internal clock uses daylight to synchronize and anticipate daily bodily routines and to adjust to the changing length of days across seasons.

Now, new research from scientists at the University of California San Francisco and the University of Copenhagen shows that our diet may also play a crucial role in the biological clock’s ability to adapt to summer and winter.

In a study on mice, researchers examined how different types of fats affected their internal clock.

“It has long been believed that only changes in the length of daylight regulate the adaptation of our biological rhythms to the change of seasons,” says Professor Thomas Mandrup-Poulsen from the Department of Biomedical Sciences at the University of Copenhagen, who contributed to the study recently published in the journal Science.

“But in our study, we found that mice fed a ‘summer diet’ – high in calories or low in polyunsaturated fatty acids – adapted more quickly to a simulated ‘summer’ diurnal rhythm, where such diets are naturally prevalent, but more slowly to ‘winter’, when diets are typically low in energy and rich in polyunsaturated fats,” he adds.

For example, mice on the summer diet adjusted more quickly to waking up and being active at the appropriate time during the simulated summer.

It may be advantageous for the mouse with multiple signals helping the biological clock adapt to the seasons – for example when food is scarce.

“If a harsh winter is followed by an extremely dry spring with poor plant growth, the mouse will not encounter the usual increase in availability of foods with high energy density and saturated fat. The biological clock picks up on this signal, telling the mouse to wake earlier to spend more time searching for food to compensate for lack of food,” says Thomas Mandrup-Poulsen.

We eat like it's summer all year

Humans also have an internal clock that functions similarly to that of mice. This raises the question of whether modern dietary patterns adversely affect our circadian rhythms.

Unlike mice, most people have access to the same types of food year-round, and we tend to eat energy- and fat-rich foods – as if it were summer all year round.

Researchers believe this could create a mismatch between the seasonal light signals our internal clock receives from the sun and the signals received from our diet.

“Our work shows that seasonal variation in the availability of polyunsaturated fatty acids relative to saturated fatty acids in the diet affects signals to the clock, including the phosphorylation of PER2. This could mean that consuming foods low in polyunsaturated fatty acids year-round could lock people into a persistent ‘summer circadian rhythm’,” says lead author Daniel Levine, postdoctoral fellow at the Department of Neurology, University of California San Francisco.

“During winter, a mismatch may occur between shorter light exposure and food intake that signals summer. This desynchronization of two key stimuli for the internal clock could be problematic,” adds Thomas Mandrup-Poulsen.

Biological jetlag may lead to lifestyle diseases

It’s well known that disruption of  the biological clock – such as shift work –  increases the risk of various lifestyle diseases, explains Thomas Mandrup-Poulsen. And the mismatch between dietary and light signals may lead to a kind of biological jetlag.

Biological jetlag could contribute to disrupted appetite regulation, potentially leading to obesity and subsequently metabolic, cardiovascular, and psychiatric disorders.

“If our body, due to our diet, thinks it’s summer when it’s actually winter, we may consume and store more energy – like if a squirrel could hoard food year-round. This biological jetlag could contribute to disrupted appetite regulation, potentially leading to obesity and subsequently metabolic, cardiovascular, and psychiatric disorders,” he says.

The next step is to investigate whether consuming more seasonal foods with energy density and fat composition determined by the season can improve our biological clock’s seasonal adaptation.

“Mice are forced to eat local and seasonal food, but humans can go to the supermarket year-round and buy energy-rich food from all over the world. What would happen if supermarkets only offered seasonal, locally grown foods – like the Mediterranean diet or the new Nordic cuisine? We’d likely be more in sync with the food we consume and the light we experience. And it would even be more environmentally sustainable,” says Thomas Mandrup-Poulsen.

Read the study ”Unsaturated fat alters clock phosphorylation to align rhythms to the season in mice”.